The present invention relates to a cathode for use in a picture tube, a manufacturing method therefor, and to a picture tube using the cathode.
Conventional cathodes for color picture tubes are often oxide cathodes of a base metal coated with an oxide of an alkali earth metal such as barium, strontium, or calcium serving as the electron-emitting material. Methods frequently used for coating the base metal with the electron-emitting material include spraying a suspension of the electron-emitting material in a binder such as nitrocellulose or ethylcellulose onto the base metal.
FIG. 11 is a schematic view of a conventional oxide cathode. As is shown in FIG. 11(a), the cathode structure includes a cylindrical sleeve 1, a base metal 2 covering one of the aperture portions of the sleeve 1, and an electron-emitting layer 4 formed on the base metal 2. Usually, the electron-emitting layer 4 has homogeneous porosity and a suitable density for electron emission. To provide the electron-emitting layer 4 with suitable and homogeneous porosity, it is preferable that the average particle size of the crystal particles in the electron-emitting layer 4 is at least 5 xcexcm. Here, average particle size of the crystal particles in the electron-emitting layer 4 means the average particle size of the electron-emitting crystals solidified from the binder suspension. When the average particle size of the crystal particles is more than 5 xcexcm, the planarity of the surface of the electron-emitting layer 4 (electron-emitting surface) becomes low, as is shown in FIG. 11(b).
FIG. 12 shows an electrical equipotential distribution near the electron-emitting surface and a current-density distribution of the electrons emitted from the electron-emitting surface, when the planarity of the electron-emitting surface is low. Numeral 7 in FIG. 12 indicates a control electrode, and numeral 8 indicates an acceleration electrode. When the electron-emitting surface of the electron-emitting layer 4 has a low planarity and many irregularities, the electrical equipotential distribution 5 that causes the electron emission and is formed in front of the electron-emitting surface warps, and so does the current density distribution 6 of the electrons emitted from the electron-emitting surface, as is shown in FIG. 12.
When the current density distribution 6 of the electrons emitted from the electron-emitting surface warps, distortions in the brightness distribution of the electron beam spot formed on the fluorescent screen of the picture tube may occur. It is well-known that these distortions in the brightness distribution of the electron beam may result in moirxc3xa9 caused by the interference of the phosphorous dot arrangement and the scanning beam.
A picture tube cathode with improved planarity of the electron-emitting surface is known from Publication of Unexamined Japanese Patent Application No. Hei 5-74324. This cathode is explained with reference to FIG. 13. As is illustrated in FIGS. 13(a) and 13(b), an electron-emitting layer 9 is divided into two layers, namely a lower layer 10 adhering to the base metal 2 and an upper layer 11 formed on top of the lower layer 10. The average particle size of the electron-emitting material of the upper layer 11 is smaller than the average particle size of the electron-emitting material of the lower layer 10. By selecting an average particle size of 5 to 20 xcexcm (for example 10 xcexcm), the lower layer 10 can be made porous and a suitable density for electron emission can be realized. Moreover, by selecting an average particle size of less than 5 xcexcm (for example 3 xcexcm), the planarity of the surface of the electron-emitting upper layer 11 is improved.
However, when the electron-emitting layer is formed with this conventional technique, electron-emitting materials of two different particle sizes are necessary. And, when an electron-emitting material of less than 5 xcexcm average particle size is used to form the upper layer of the electron-emitting layer, the porosity of the upper layer surface is lost, and it becomes difficult to attain a desired electron emission.
The present invention has been developed to overcome the problems of the prior art. It is a purpose of the present invention to provide a cathode with improved planarity of the surface of the electron-emitting layer and smooth current density distribution for the electrons emitted from the electron-emitting surface, without deterioration of the electron emission characteristics. It is another purpose of the present invention to provide a method for manufacturing such a cathode, and a picture tube using this cathode.
In order to achieve these purposes, a cathode in accordance with the present invention comprises a base metal and an electron-emitting layer of electron-emitting material formed on the base metal. A surface of the electron-emitting layer is mechanically flattened after spraying the electron-emitting material onto the base metal. In such a cathode, a porous structure is formed throughout the entire electron-emitting layer, so that a certain electron emission can be attained, the planarity of the electron-emitting surface can be improved, and the current density distribution of the electrons emitted from the electron-emitting surface can be smoothened.
It is preferable that the cathode according to the present invention further comprises an adhesive coating between the base metal and the electron-emitting layer. In this preferable example, a decrease in the adhesive force between the base metal and the electron-emitting layer caused by mechanical flattening (for example by pressing) of the electron-emitting surface can be prevented.
It is preferable that in the cathode according to the present invention, the surface of the electron-emitting layer is flattened only in a region comprising an electron-emitting region. In this preferable example, a decrease in the adhesive force between the base metal and the electron-emitting layer caused by mechanical flattening (for example by pressing) of the electron-emitting surface can be prevented.
It is preferable that the surface roughness of the surface of the electron-emitting layer (maximum height Ry in JIS B 0601) in the cathode of the present invention is not more than 15 xcexcm. In this preferable example, the current density distribution of the electrons emitted from the electron-emitting surface can be smoothened.
A method for manufacturing a cathode comprising a base metal and an electron-emitting layer of electron-emitting material formed on the base metal in accordance with the present invention comprises the steps of spraying the electron-emitting material onto a metal base to form the electron-emitting layer; and mechanically flattening an electron-emitting surface of the electron-emitting layer.
It is preferable that the method for manufacturing a cathode according to the present invention further comprises a step of injecting an adhesive coating between the base metal and the electron-emitting layer, after flattening the electron-emitting surface.
A picture tube in accordance with the present invention comprises a face panel having a phosphorous screen on an inside surface; a funnel connected to the rear of the face panel; an electron gun having a cathode in a neck portion of the funnel, the cathode comprising a base metal and an electron-emitting layer of electron-emitting material formed on the base metal, wherein a surface of the electron-emitting layer is mechanically flattened after spraying the electron-emitting material onto the base metal. As a result, the moirxc3xa9 caused by interference of the phosphorous dot arrangement and the electron scanning beam can be decreased.
It is preferable that the cathode in the picture tube according to the present invention further comprises an adhesive coating between the base metal and the electron-emitting layer.
It is preferable that the surface of the electron-emitting layer in the picture tube of the present invention is flattened only in a region comprising an electron-emitting region.
It is preferable that the surface roughness of the surface of the electron-emitting layer (maximum height Ry in JIS B 0601) in the picture tube of the present invention is not more than 15 xcexcm.